my sportsman 500 keeps overheating all the time. I have tested the fan, it works. I have replaced the temp sensor switch. I have replaced the thermostat, and replaced the head gasket also. I am starting to lose my mind I cant figure this out.
I start my wheeler up and run it, but the fan doesnt come on before the antifreeze starts to boil. the quad get hot, I feel the metal on the the temp sensor switch and its cold. Its like the antifreeze isn't circulating. I then tried taking the t-stat out and running it. Still the wheeler overheating.

I have a 2004 Sportsman 400 doing the same thing. It overheats and the fan wont kick on. I had an issue with the radiator cap, the fluid was boiling, replaced the cap and cured that problem. But the thing still over heats, I hard wired the fan and I can ride a little longer but still not right!

You said you changed head gasket...what about the base gasket?
Was it overfilling the resevoir bottle? It usually will if you have a pressure leak in the cylinder. Then radiator gets low and then its hot. Fan won't come on if coolant not on the sensor switch.
You also have to purge the air out of system after change.
If coolant not circulating its the pump.
If you got circulation and the fan don't kick on but coolant is boiling you probably have something wrong with either the sensor, the fan, or the wiring. You can try wiring the fan up with a switch to see if that keeps it cool.
You might also check the color of the spark plug to see if running too lean but that would have nothing to do with fan not coming on.

2000 sportsman 500 I am having the same problem. Have had the machine for 5 years never had a problem, took it down the railroad bed about ten miles light came on got back to my truck it was leaking fluid out the seep hole by water pump. Took it to a dealer replaced the mechanical seal behind the water pump presure tested it said it was good to go went down same railroad bed light came on after a couple of miles fan never came on. Brought it home replaced temp sensor in radiator new thermos stat cleaned radiator. The top of the radiator gets hot bottem stays cool. I know the fan works. Put differant cap on radiator same problem. I don't understand what is going on here. The system seems to not be circulating. Help i want to ride

2000 sportsman 500 I am having the same problem. Have had the machine for 5 years never had a problem, took it down the railroad bed about ten miles light came on got back to my truck it was leaking fluid out the seep hole by water pump. Took it to a dealer replaced the mechanical seal behind the water pump presure tested it said it was good to go went down same railroad bed light came on after a couple of miles fan never came on. Brought it home replaced temp sensor in radiator new thermos stat cleaned radiator. The top of the radiator gets hot bottem stays cool. I know the fan works. Put differant cap on radiator same problem. I don't understand what is going on here. The system seems to not be circulating. Help i want to ride

Are you assuming, because the dealer did the work, that the water pump is good? Or do you know that it is good?

I've got one idea. A couple years ago, I had a similar problem, and went to the dealership 3 times for it, until finally figured out a solution with the mechanic.

This may or may not be your problem. But just in case, CLEAN that radiator! In my 700 the fins just weren't clean enough. So, I've started using Eagle One Mag Cleaner for alloy wheels. Spray directly into the fins, and let it sit a few minutes. I was very surprised to see the amount of dirt that came out when I flushed it out.

don't get it on painted areas or plastic! it's some rough stuff. Maybe this will help.

Fixed it last night!! Went to different mechanic talked to him told him everything i did he didn't have any other suggestions. Just when i fill it with fluid go slow and bleed it with the bolt at the water pump. Took off the water pump housing cover to check it one more time found i could turn the impeller with my finger!!! Tightened nut up put it back together problem solved. Just goes to show you sometimes you need to check other peoples work. Hope this helps anybody with same problem. So after an extra $150 and hours of frustration all it was a loose nut that should have been tight from the dealer. Good call Phil